feat(core): adaptive upload parallelism with throughput probe and disk-saturation gate

Close the control loop around the in-flight-file count (DESIGN 11.4.7 /
18.2). A 30s-window ThroughputProbe fed by the executor drives a resizable
UploadPool within [1, 32]: shrink when a window's throughput collapses
below 50% of the prior one (and neither the pacer nor the disk explains the
drop), grow while lifting the pool still improves throughput. A per-OS
disk-busy gate (new driven-diskstat crate: PDH on Windows, /proc/diskstats
on Linux, IOKit on macOS; fail-open to not-saturated so a broken reader
never strangles uploads) blocks growth when the disk is the bound.

Default-on with an adaptive_parallelism_enabled kill-switch; off = today's
fixed pool at default_concurrent_uploads (now wired as the start size). The
control law is a pure, exhaustively unit-tested decide(); an e2e_fake test
asserts a real shrink-then-regrow transition through the shared
executor/probe/pool wiring.

Refs #34

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QZQVP2tUuTLh8oL31D8heC

Author: pmaxhogan

Cargo.lock

@@ -5,6 +5,7 @@ members = [
     "crates/driven-drive",
     "crates/driven-crypto",
     "crates/driven-power",
+    "crates/driven-diskstat",
     "crates/driven-vss",
     "crates/driven-vss-helper",
     "crates/driven-net",

Cargo.toml

@@ -47,6 +47,10 @@ sqlx = { version = "0.9", default-features = false, features = [
 driven-drive = { path = "../driven-drive" }
 driven-crypto = { path = "../driven-crypto" }
 driven-power = { path = "../driven-power" }
+# Per-OS disk-busy reader for the adaptive upload-parallelism controller
+# (DESIGN s11.4.7 / s18.2). Same acyclic shape as driven-power: it carries no
+# driven-core dep.
+driven-diskstat = { path = "../driven-diskstat" }
 # M3.5 Windows VSS reads for exclusively-locked files (ROADMAP M3.5, DESIGN
 # s5.3). The executor consults the `VssProvider` seam on its open path; the
 # orchestrator owns the per-cycle snapshot lifecycle + orphan cleanup. The